Taking variations of both sides of Eq. (REF ) over \(\mu \)  with the
help of
Eqs. (REF ) and (REF ) and changing the order of the integral
over the phase space \(\Gamma \)  and derivative over the chemical potential \(\mu \) ,
at first order variations, i.e., the second order in fluctuations one obtains
(see Refs. [1]}, [2]})
\(\omega (n,T) =\frac{\langle \left(\Delta N \right)^2\rangle }{\langle N \rangle }=T\frac{(\delta n/\delta \mu )^{}_T}{n}~,\) 
